To update this thread:
I built it as stated, 5' of 6 In. schedule 80, 1-6X6X6 "T", 2-6" caps, a 6X2 reducer, 18" if 2" for a filler and a 2" cap. Sets in front between the bed front and the hitch.
Glued it all together with regular Purple stuff. Hasn't leaked a drop.
Drilled and threaded one end, installed a 1/4 compression fitting for the tubing, which goes to the pump under the back seat.
I will post pictures later.
Works great, holds about 8-9 gallons. Which is good for about 600 Miles pulling my 5er.
Total cost for me was $72. 00 ( I already had 10' of 6")